The present invention relates generally to machines for extrusion of materials and more particularly to screw extruders adapted for use with plastics and plastic-like materials. The inventor anticipates that primary application of the present invention will be for the manufacture of color concentrates, polymer blends, and polymer alloys or articles produced by mixing polymers with concentrates, fillers, other polymers, additives, and the like.
A screw extruder is a machine in which material, usually some form of plastic, is forced under pressure to flow through a contoured orifice in order to shape the material. Injection molding machines utilize extruders to force materials under pressure into a mold cavity. Screw extruders are generally composed of a housing, which is usually a cylindrical barrel section, surrounding a central motor-driven screw. At a first end of the barrel is a feed housing containing a feed opening through which new material, usually plastic particles, is introduced into the barrel. The screw contains raised portions called flights having a larger radial diameter than the screw""s central shaft and which are usually wrapped in a helical manner about the central shaft. The material is then conveyed by these screw flights toward the second end of the barrel through a melting zone, where the material is heated under carefully controlled conditions to melt the material, and then passes through a melt-conveying zone, also called a pumping zone. The melted plastic is finally pressed through a shaped opening or die to form the extrudate.
Besides conveying material toward the die for extrusion, the screw is depended upon to perform mixing of the feed material. Very generally, mixing can be defined as a process to reduce the non-uniformity of a composition. The basic mechanism involved is to induce relative physical motion in the ingredients. The two types of mixing that are important in screw extruder operation are distribution and dispersion. Distributive mixing is used for the purpose of increasing the randomness of the spatial distribution of the particles without reducing the size of these particles. Dispersive mixing refers to processes that reduce the size of cohesive particles as well as randomizing their positions. In dispersive mixing, solid components, such as agglomerates, or high viscosity droplets are exposed to sufficiently high stresses to cause them to exceed their yield stress, and they are thus broken down into smaller particles. The size and shape of the agglomerates and the nature of the bonds holding the agglomerate together will determine the amount of stress required to break up the agglomerates. The applied stress can either be shear stress or elongational stress and generally, elongational stress is more efficient in achieving dispersion than is shear stress. An example of dispersive mixing is the manufacture of a color concentrate where the breakdown of pigment agglomerates below a certain critical size is crucial. An example of distributive mixing is the manufacture of miscible polymer blends, where the viscosities of the components are reasonably close together. Thus, in dispersive mixing, there will always be distributive mixing, but distributive mixing will not always produce dispersive mixing.
In some extrusion processes, the need for good dispersive mixing is more important than for distributive mixing. This is particularly true in the extrusion of compounds which contain pigment agglomerate that must be reduced in size.
In screw extruders, significant mixing occurs only after the polymer has melted. Thus, the mixing zone is thought of as extending from the start of the melting zone to the end of the extrusion die. Within this area there will be considerable non-uniformities in the intensity of the mixing action and the duration of the mixing action, both in the barrel section and in the extrusion die. In molten polymer, the stress is determined by the product of the polymer melt viscosity and rate of deformation. Therefore, in general, dispersive mixing should be done at as low a temperature as possible to increase the viscosity of the fluid, and with it, the stresses in the polymer melt.
Fluid elements are spoken of as having a xe2x80x9cmixing historyxe2x80x9d, which refers to the amount of elongational and shear stress to which it has been exposed, and the duration of that exposure. A polymer element that melts early in the melting zone process will have a more significant mixing history than one that melts near the end of the melting zone.
Generally, in an extruder with a simple conveying screw the level of stress or fraction of the fluid exposed to high stresses is not high enough to achieve good dispersive mixing. Distributive mixing is easier to achieve than dispersive mixing, but unmodified screws have also been found to produce inadequate distributive mixing for many applications. Therefore, numerous variations in screw design have been attempted in prior inventions to increase the amount of distributive or dispersive mixing in screw extruders. These devices usually contain a standard screw section near the material input hopper, and one or more specially designed sections to enhance mixing. These mixing sections naturally fall into the categories of distributive and dispersive mixing elements although some mixing devices achieve both distributive and dispersive mixing.
Prior mixers that have attempted to improve distributive and dispersive mixing are shown in FIGS. 3-6 (prior art). Three mixers, the Cavity Transfer Mixer (CTM), the Twente Mixing Ring (TMR), and the Kneader, are discussed below.
FIG. 3 (prior art) shows the geometry of the CTM. It consists of a screw extension with hemi-spherical cavities and a barrel extension that also contains hemi-spherical cavities. The screw rotates and the barrel is stationary. The fluid passing through the mixer flows from a screw cavity to a barrel cavity and back to another screw cavity. This action repeats itself several times as the fluid passes through the mixer. The CTM was a significant development because it was able to improve the mixing capability of single screw extruders (SSE) significantly. The reason for the efficiency of the CTM is the multiple reorientation events that occur when the fluid moves from a cavity in the screw to a cavity in the barrel.
The CTM suffers from several practical drawbacks that have limited the commercial success of this mixer. Some of these disadvantages are:
1. The mixing section has no forward pumping capability; as a result, it is a pressure consuming element of the extruder and this will tend to reduce the extruder output and increase the polymer melt temperature.
2. The barrel has hemi-spherical cavities in the CTM section. This means that a separate CTM barrel section has to be installedxe2x80x94this increases the cost of the mixer substantially and also complicates the installation of a CTM.
3. The barrel surface is no longer completely wiped by the screw. Polymer melt will enter the barrel cavities and the polymer melt flow in the bottom of the cavities can be very slow. As a result, when a change in material is made (e.g. from white to red) it can take an inordinately long time for the old material to disappear in the extruded product. Therefore, in many cases the mixer has to be physically cleaned when a material change is made. This cleaning can be quite time consuming and results in lost production. This can be a distinct disadvantage when frequent material changes are made.
The TMR (FIG. 4, prior art) was developed at Twente University by Semmekrot. The TMR uses the same principle of mixing as the CTM; however, instead of using cavities in the barrel it uses a floating annular mixing ring or sleeve with holes bored into it. The mixing sleeve rotates with the screw but at a lower rotational speed and this provides the relative velocity between the screw cavities and the sleeve cavities. Thus, the TMR eliminates an important drawback of the CTM, the cavities in the barrel. As a result, the TMR can be used in regular extruders and IMMs without the need to add a separate barrel section with cavities. The TMR is successfully used in a number of injection molding applications where the mixer is incorporated into the non-return valve. Since the typical non-return valve (NRV) is relatively short the mixing action of a mixing action of a mixing NRV has to be very effective to produce a product with good homogeneity.
FIGS. 5-6 (prior art) show the Buss Ko-Kneader. This is a single screw compounding extruder where the screw rotates and reciprocates axially. The barrel of the kneader is equipped with three axial rows of mixing pins and the screw flights have slots machined in them so that the pins move through the slots of the screw flights. This creates a very efficient mixing action that allows the kneader to be much shorter than conventional compounding extruders. The typical length of a kneader is about 11D while the typical length of a twin screw compounder is about 35D-50D. The efficient mixing action of the kneader is created by the barrel pins that cause a much more efficient surface generation than a smooth barrel surface. However, the installation of pins in the inner barrel creates expense, as well as the difficulty of introducing the screw into the barrel without encountering the pins.
U.S. Pat. No. 6,305,831 shows an apparatus for mixing a polymer melt that uses a hollow outer shaft having mixing elements attached to the inner and outer surfaces in conjunction with mixing elements on the inner shaft to mix material. The hollow outer shaft is driven by a gear wheel and is apparently not available to retrofit within regular screw extruders or plasticating units.
For the foregoing reasons, there is a great need for a screw extruder which provides better distributive and dispersive mixing than in presently available extruders, which utilizes a floating ring having mixing elements that interact with mixing elements on the main screw and which can travel at a different velocity than the main screw, and which is retrofitable within standard extruders.
Accordingly, it is an object of the present invention to provide a mixing section which provides improved mixing.
Another object of the invention is to provide a mixing section which is usable in both screw extruders and injection molding machinery.
And another object of the invention is to provide a mixing section which provides excellent mixing and is simple to manufacture.
A further object of the present invention is to provide a mixing section that is easily retrofitable to many existing types of screw extruder barrels.
An additional object of the present invention is to provide a mixing section which produces a low pressure drop in this mixing section.
Yet another object of the present invention is to provide a mixing section that provides a high number of re-orientation events for the material being processed.
A yet further object of the present invention is to provide a mixing section in which there is good streamlining of material, a large number of divisions per cross-section and the barrel of the extruder is completely wiped.
Briefly, one preferred embodiment of the present invention is a mixing section for mixing material in an extruder having a barrel. The mixing section includes a screw having a central shaft, and a floating annular sleeve located between the screw and the barrel, and surrounding a portion of the screw. The sleeve has mixing elements protruding radially inward toward the screw. A portion of the central shaft has mixing elements protruding radially outward toward the sleeve. The mixing elements on the screw and the annular sleeve create multiple regions of reorientation in the material to be mixed which produce improved distributive mixing.
Also disclosed are an extruder having such a mixing section and a floating sleeve for use in a mixing section.
An advantage of the present invention is that material change-over time between runs of materials of different colors or compositions is very fast.
Another advantage of the present invention is that the present mixing section produces a very high number of re-orientation events, and thus very high quality distributive mixing of materials.
And another advantage of the present invention is that there is very little pressure drop in the mixing section, thus throughput is very high.
A further advantage of the present invention is that the main transfer direction of material is axial, rather than radial, so throughput can be maintained at a high level.
A yet further advantage is that the present mixing section can be retrofitted into existing extruders and injection molding machines at a relatively low cost.
An additional advantage is that because there is such a high concentration of re-orientation events in the mixing section, the overall length of the mixer can be reduced.
A further advantage is that the quality of mixing is so high that it rivals that of multiple screw mixers, which are much more expensive, thus providing cost benefits to manufacturers.